Membrane separation, which uses a selective membrane, is an increasingly common method of processing of liquid streams, such as in water purification. In membrane separation, constituents of the influent typically pass through the membrane as a result of a driving force(s) in one effluent stream called permeate thus leaving behind some portion of the original constituents in a second stream called concentrate or reject. Membrane separations commonly used for water purification or other liquid processing include the pressure driven processes of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). The performance of membrane separation, such as efficiency (e.g. flux, membrane permeability, permeate recovery, permeate quality, energy efficiency, time between membrane cleanings or time to conduct a cleaning cycle) and effectiveness (e.g. rejection or selectivity) are typically reduced by membrane fouling.
Membrane separation processes are prone to fouling by microbes, i.e. biofouling, particularly in aqueous streams, which provide optimum conditions for microbial growth. Biofouling is particularly detrimental to membrane separation systems because once it is started, the growth rate accelerates and biofouling can facilitate other types of fouling. For example, the extra-cellular polymeric substances (“EPS”) or slime layer of the biomass can trap and hold scales and other particulates that might otherwise pass out of the membrane separation system during operation. Furthermore, a thick EPS layer can also decrease turbulent flow within the membrane. This can lead to an increase in the concentration polarization layer, which is a known contributor to membrane scaling phenomena.
A review article written by H. F. Ridgway & H. Flemming entitled “Membrane Biofouling in Water Treatment Membrane Processes”, McGraw Hill, pp. 6.1 to 6.62, 1996, discusses many aspects of membrane fouling.
In general, biofouling is controlled through the use of biocides and other biocontrol agents, and by periodic cleaning of membrane elements/systems to remove the biofouling and associated debris.
Membrane fouling (colloidal, organic, bio, or a combination thereof) and scaling, can adversely impact the membrane performance. For example, it can decrease the permeate flow at a given driving force, lower the permeate quality (purity), and increase energy consumed to maintain a given permeate flow. This can necessitate the cleaning of the membrane separation system in order to remove the deposits.
Membrane fouling occurs in systems that treat water, wastewater, or industrial process streams.
Membrane cleaning processes usually consist of removing the membrane system from service, rinsing the membrane system with high quality water, preparing a cleaning solution, heating the cleaning solution, circulating the cleaning solution at low pressure through the membranes and back into the clean-in-place (CIP) tank. The process may also include alternating periods of circulating the cleaning solution through the system and soaking the system in the cleaning solution. The system may also be rinsed and fresh cleaning solution applied as needed. Finally the system is rinsed with permeate quality water and either subjected to a second cleaning or placed back in service.
Improved cleaning methods and products are needed because they can enhance the performance of the membrane separation process. Examples of such optimized performance include longer times between membrane cleanings, longer membrane life, and decreased energy costs due to better control of scaling, fouling, and other system parameters.